1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring the size and direction of a velocity vector of a relatively moving object or the position of the object by use of a spatial filter.
2. Description of the Prior Art
A spatial filter having a row of slits formed on the surface thereof at a predetermined pitch is employed to detect a frequency of light transmitted through the spatial filter and measure the velocity of a moving object that is emitting the light. When the object to be measured, which may be a two-dimensional pattern or a texture, moves at a velocity of V above the spatial filter having the slits of pitch P, the light from the object transmitted through the spatial filter is collected and photoelectrically converted into electric signals. Supposing a basic frequency of the electric signals is f, the following is established: EQU V=f.multidot.P (1)
Based on this equation, the velocity V of the object can be found. Namely, when a light spot is moving at a velocity of V behind a grid having a pitch of P, a flickering frequency of the light spot in front of the grid is f. Since the pitch P of the grid is known and the flickering frequency f can be measured by a frequency analyzing method, etc., the velocity V of the light spot may be calculated and measured. This is described in, for example, "Spatial Filter and Its Application (I)," in a journal "Measurement and Control" Vol. 19, No. 4, April, 1980. The velocity measurement by the spatial filter technique employing the spatial filter is simple and sure because it can uniquely determine a velocity according to the equation (1). This is why the technique is widely practiced in the steel and paper industries for measuring velocities of steel plates and sheets, in the real-time measurement of a traveled distance of a marathon runner, in the velocity measurement of a super high-speed plasma-like flying particle, or in the wind velocity measurement in which a temperature distribution is used as a kind of pattern.
The conventional velocity measurement employing the spatial filter is capable of measuring the size of a velocity vector, but however, incapable of measuring the direction of the velocity vector. Namely, the conventional spatial filter technique reacts similarly to a pattern moving from the right to the left in front of a spatial filter grid and to the same pattern moving from the left to the right. Accordingly, it is impossible to measure the direction of the movement of the pattern. It is generally recognized, therefore, that the conventional spatial filter technique is useful only for accurately measuring the size of a velocity vector but useless for measuring the direction of the velocity vector. This problem limits the application of the spatial filter technique, and must be resolved.